AU4781700A

AU4781700A - Production device and method for optical fiber

Info

Publication number: AU4781700A
Application number: AU47817/00A
Authority: AU
Inventors: Kazuya Kuwahara; Katsuya Nagayama; Yuichi Ohga; Tatsuhiko Saito
Original assignee: Sumitomo Electric Industries Ltd
Current assignee: Sumitomo Electric Industries Ltd
Priority date: 1999-05-27
Filing date: 2000-05-26
Publication date: 2000-12-18
Anticipated expiration: 2020-05-26
Also published as: US20060101861A1; AU773667B2; EP1205449B1; CN1247477C; EP1205449A1; WO2000073224A1; EP1205449A4; CN1350509A

Description

FPO0-0085-00 DESCRIPTION Optical Fiber Making Apparatus and Method Technical Field 5 The present invention relates to an apparatus andmethod for making an optical fiber whose transmission loss is reduced by lowering Rayleigh scattering intensity. Background Art As a method of making an optical fiber whose 10 transmission loss is reduced by lowering Rayleigh scattering intensity, one disclosed in Japanese Patent Application Laid-Open No. HEI 10-25127 has been known, for example. This method comprises the steps of producing an intermediate optical fiber by drawing an optical f iber preformupon heating, 15 and heat-treating the intermediate optical fiber by reheating, so that the reheating causes structural relaxation (atomic rearrangement) in glass, which lowers the fictive temperature (temperature to which the disorder in the state of atomic arrangement within the glass corresponds) , thereby reducing 20 Rayleigh scattering intensity. Disclosure of the Invention However, for protecting the optical fiber drawn upon heating, the surface of optical fiber immediately after drawing has been coated with a UV resin or the like. Since 25 the resin coated on the optical fiber may be burned by the heat at the time of reheating, the above-mentioned method 1 * FP00-0085-00 of making an optical fiber disclosed in Japanese Patent Application Laid-Open No. HEI 10-25127 is not suitable for mass-producingoptical fiber. Thoughtheoptical fibersmay be reheated in a state where its surface is not coated with 5 a resin, this method is not employable as a method for mass production due to damages occurring at the time of handling the optical fiber, and the like. In view of the above-mentioned points, it is an object of the present invention to provide, when making an optical 10 fiberwhose transmission loss is loweredbyreducingRayleigh scattering intensity, an apparatus and method for making an optical fiber, which is applicable to the mass production of optical fiber whose surfaces are coated with a resin. The inventors carried out diligent studies about an 15 apparatus and method for making an optical fiber which is applicable to the mass production of coated optical fiber and, as a result, have newly found the following fact concerning the Rayleight scattering intensity and the cooling rate of the optical fiber after drawing. 20 Within glass at a higher temperature, atoms are vigorously vibrating due to thermal energy, whereby the atomic arrangement is in a state more disordered than in glass at a lower temperature. If the glass at a higher temperature is slowly cooled, the atoms within a temperature 25 range where they are allowed to be rearranged are cooled while being arranged with the level of disorder corresponding 2 FPOO-0085-00 each temperature, whereby the level of disorder of the atoms within the glass attains the state corresponding to the lowest temperature (about 1200*C) at which the structural relaxation proceeds. If glass at ahigher temperature is cooledrapidly, 5 the atomic arrangement is fixed upon cooling before reaching an equilibrium state corresponding to each temperature, thus yielding amore disordered state than in the case of annealing. Even in the same material, the Rayleigh scattering intensity becomes greater when the atomic arrangement is more 10 disordered. Optical fibers usually cooled at a cooling rate of 5000 to 30000*C/s after drawing are in a state yielding a more disordered atomic arrangement and a higher fictive temperature as compared with bulk glass, which is supposed to be a reason why Rayleigh scattering intensity is greater. 15 On the other hand, the time required for structural relaxation becomes longer as temperature is lower, whereby the structural relaxation does not occur at a temperature of about 12000C unless this temperature is held for several tens of hours, for example. Since the optical fiber after 20 drawing is normally cooled from about 20000C to about 4000C within several hundreds of milliseconds, it is necessary that the annealing be effected in a state at a temperature higher than 1200 0 C in order for the 'fictive temperature to be lowered to approach 12000C within a short period of time 25 during which the optical fiber in its drawing step is cooled. Therefore, taking account of the optical fiber 3 FP00-0085-00 temperature and cooling rate after drawing, the inventors investigated the relationship between the cooling rate and Rayleigh scattering coefficient in a part of optical fiber where the temperature was within the range from 1200 to 1700*C, 5 which was higher than the lowest temperature (about 1200*C) at which the above-mentioned structural relaxation proceeded but not exceeding 1700 0 C at which the structural relaxation proceeded within a very short period of time. As a result, it has been verified that the relationship shown in Fig. 10 8 exists between the cooling rate and Rayleigh scattering coefficient in a part of optical fiber where the temperature is 1200 to 17000C. Here, Rayleigh scattering intensity (I) has such a characteristic as to be inversely proportional to the biquadrate of wavelength (X) as shown in the following 15 expression (1)

I=A/

4 (1) where coefficient A is defined as Rayleigh scattering coefficient. From these results, it has been found that, if the 20 cooling rate in a predetermined segment in an optical fiber drawn upon heating before being coated with a resin, in particular in a part of optical fiber where the temperature is 1200 to 17000C, is lowered, then the Rayleigh scattering intensity of optical fiber is reduced, whereby the 25 transmission loss can be lowered. In view of such results of studies, for achieving the 4 FPOO-0085-00 above-mentioned object, the present invention provides an apparatus for making an optical fiber for drawing an optical fiber preform upon heating and coating thus drawn optical fiber witha resin; the apparatus comprising a heating furnace, 5 disposed between a drawing furnace for drawing the optical fiber preform upon heating and a resin coating section for coating the drawn optical fiber with the resin, for heating the drawn optical fiber such that the optical fiber attains a temperature within the range from 1200 to 1700*C, theheating 10 furnace having a muffle tube through which the drawn optical fiber passes, the muffle tube being disposed at a position satisfying: L1!0.2xV where 15 Ll is the distance (m) from the lower end of a heater in the drawing furnace to the upper end of the muffle tube; and V is the drawing rate (m/s). Since the heating furnace for heating the drawn optical 20 fiber such that the optical fiber attains a temperature within the range from 1200 to 17000C is disposed between the drawing furnace and resin coating section, the cooling rate is lowered in a predetermined segment of the part of optical fiber where the temperature is 1200 to 17000C in the optical fiber drawn 25 upon heating before being coated with the resin, whereby this part is annealed. Therefore, the optical fiber lowers 5 FP00-0085-00 its fictive temperature, whereby it becomes possible to make an optical fiber having decreased its transmission loss by reducing its Rayleigh scattering intensity during a period from the drawing upon heating to the resin coating. Also, 5 since the Rayleigh scattering intensity is reduced by controlling the cooling rate of optical fiber after drawing before being coated with a resin, the heat treatment for reheating such as that in the above-mentioned prior art is unnecessary, whereby the apparatus is quite easily applicable 10 to the mass production of optical fiber whose surfaces are coated with a resin. In the case where the drawing rate is higher, the position where the drawn optical fiber attains a given temperature shifts toward the resin coating section as 15 compared with that in the case where the drawing rate is lower. Therefore, if themuffle tube is located at aposition satisfying L1 0.2xV , then it canbeplacedat an appropriate position corresponding 20 to the level of drawing rate, whereby the cooling rate of optical fiber can be lowered appropriately. In the apparatus for making an optical fiber in accordance with the present invention, the heating furnace may heat the drawn optical fiber at a temperature within 25 the range from 1200 to 16000C. When the heating furnace heats the drawn optical fiber 6 FP00-0085-00 at a temperature within the range from 1200 to 1600 0 C, the cooling rate of the optical fiber lowers in a predetermined segment in the part of optical fiber where the temperature is 1200 to 1700 0 C, so that the fictive temperature of the 5 optical fiber decreases, whereby the Rayleigh scattering temperature can further be reduced. Here, the temperature of heating furnace is that near the furnace center. For example, the temperature of heater is set to about 1700 0 C in order for the temperature near the furnace center to become 10 about 1600 0 C. In the apparatus for making an optical fiber in accordance with the present invention, the muffle tube may be disposed at a position where the drawn optical fiber attains a temperature within the range from 1400 to 1800 0 C when 15 entering the muffle tube. If the muffle tube is disposed at a position where the drawn optical fiber attains a temperature within the range from 1400 to 1800 0 C when entering the muffle tube, then it is placed at an appropriate position corresponding to the 20 level of drawing rate, whereby the cooling rate of optical fiber can be lowered appropriately. In the apparatus for making an optical fiber in accordance with the present invention, the muffle tube may be formed so as to satisfy: 25 L2 V/8 where 7 FP00-0085-00 L2 is the total length (m) of the muffle tube; and V is the drawing rate (m/s). When the total length L2 of the muffle tube satisfies L2aV/8, 5 it can be set to an appropriate length corresponding to the level of drawing rate, whereby the cooling rate of the optical fiber can be lowered appropriately. In the apparatus for making an optical fiber in accordance with the present invention, the heating furnace 10 may be provided with such a temperature gradient that the drawing furnace side and the resin coating section side attain higher and lower temperatures, respectively. The optical fiber drawn upon heating has such a temperature distribution that the temperature decreases from 15 the drawing furnace side to the resin coating section side. Therefore, when the heating furnace is provided with such a temperature gradient that the drawing furnace side and the resin coating section side attain higher and lower temperatures, respectively, the heating furnace exhibits 20 a temperature distribution corresponding to the optical fiber having the temperature distribution mentioned above, whereby the optical fiber can be cooled at a more appropriate cooling rate. In view of the above-mentioned results of studies, for 25 achieving the above-mentioned object, the present invention provides a method of making an optical fiber comprising the 8 FP00-0085-00 steps of drawing an optical fiber preform upon heating and coating thus drawn optical fiber with a resin; wherein a segment of optical fiber yielding a temperature difference of at least 50 0 C in a part where the optical fiber before 5 being coated with the resin attains a temperature of 1300 to 1700*C is cooled at a cooling rate of 1000*C/s or lower. When the cooling rate in the segment of optical fiber yielding a temperature difference of at least 50 0 C in a part where the optical fiber before being coated with the resin 10 attains a temperature of 1300 to 1700 0 C is set to 1000 0 C/s or lower, the fictive temperature of optical fiber lowers, so that the disorder in atomic arrangement is reduced, whereby it is possible to make an optical fiber having lowered its transmission loss by reducing the Rayleigh scattering 15 intensity within a very short period from the drawing upon heating to the resin coating. Also, since the Rayleigh scattering intensity is reduced by controlling the cooling rate of optical fiber after drawing before being coated with a resin, the heat treatment for reheating such as that in 20 the above-mentioned prior art is unnecessary, whereby the method is quite easily applicable to the mass production of optical fiber whose surfaces are coated with a resin. The method of making an optical fiber in accordance with the present invention may use, as the optical fiber 25 preform, an optical fiber preform having a core portion exhibiting a relative refractive index difference of 0.001 9 FPOO-0085-00 or less with respect to pure silica glass in a state containing a dopant, and draw the optical fiber preform upon heating. Since the core portion is made of substantially pure silica glass exhibiting a relative refractive index 5 difference of 0.001 or less with respect to pure silica glass in a state containing a dopant, the optical fiber further lowers its fictive temperature, whereby it is possible to furtherlower theRayleighscatteringintensity. Here, pure silica glass in each claim refers to silica glass containing 10 no dopant, whereas the relative refractive index difference is defined by the following expression (2): relative refractive index difference = n1- refractive index of object to be comparedi/nl -- (2) where nl is the refractive index of pure silica glass. 15 The method of making an optical fiber in accordance with the present invention may use, as the optical fiber preform, an optical fiberpreformwhosecoreportionis caused to contain hydroxyl group so as to yield a transmission loss of 0.02 to 0.5 dB/km due to hydroxyl group absorption at 20 a wavelength of 1.38 pm, and draw the optical fiber preform upon heating. When hydroxyl group is contained such that the transmission loss caused by hydroxyl group absorption at a wavelength of 1.38 pm becomes at least 0.02 dB/km, the 25 optical fiber further lowers its fictive temperature, whereby the Rayleigh scattering intensity can further be reduced. 10 FPOO-0085-00 When hydroxyl group is contained such that the transmission loss is greater than 0.5 dB/km, on the other hand, the loss increases due to the absorption by hydroxyl group, which cancels out the effect of reducing Rayleigh scattering 5 intensity caused by the addition of hydroxyl group, whereby the transmission loss as a whole increases. Therefore, when the core portion is caused to contain hydroxyl group so as to yield a transmission loss of 0.02 to 0.5 dB/km due to hydroxyl group absorption at a wavelength of 1.38 pm, the 10 optical fiber further lowers its fictive temperature, whereby the Rayleigh scattering intensity can further be reduced. The method of making an optical fiber in accordance with the present invention may use, as the optical fiber preform, anoptical fiberpreformwhose coreportion is caused 15 to contain Cl so as to yield a relative refractive index difference of 0.0001 to 0.001 with respect to pure silica glass, and draw the optical fiber preform upon heating. When Cl is contained such that the relative refractive index difference with respect to pure silica glass is at 20 least 0.0001, the optical fiber further lowers its fictive temperature, whereby the Rayleigh scattering intensity can further be lowered. When Cl is contained such that the relative refractive index difference exceeds 0.001, the Rayleighscatteringintensityis enhancedbyCl itself, which 25 cancels out the effect of lowering Rayleigh scattering intensity caused by the addition of Cl, whereby the total 11 FPOO-0085-00 transmission loss increases. Therefore, when Cl is contained in the core portion such that the relative refractive index dif ference with respect to pure silica glass is 0.0001 to 0.001, the optical fiber further lowers its 5 fictive temperature, whereby the Rayleigh scattering intensity can further be lowered. The method of making an optical fiber in accordance with the present invention may use, as the optical fiber preform, an optical fiber preform having a cladding portion 10 whose part extending to the outermost periphery from a position where the distance from the center of the optical fiber preform is within the range from 0.7 to 0.9 in terms of the ratio with respect to the radius of the optical fiber preform is made of highly pure silica glass, and draw the 15 optical fiber preform upon heating. As a result of the inventors' studies, it has become clear that the Rayleigh scattering intensity increases when the tension acting on the optical fiber upon drawing is enhanced, whereas the Rayleigh scattering intensity does 20 not change when the outermost layer of optical fiber preform unrelated to light transmission is made of highly pure silica glass even if the above-mentioned tension is enhanced. When the part related to light transmission is made of highly pure silica glass, the refractive index changes, thereby 25 modifying characteristics of the optical fiber. While the part unrelated to light transmission is a part where the 12 FPOO-0085-00 distance from the center of the optical fiber preform is at least 0.7 in terms of the ratio with respect to the radius of optical fiber preform, the transmission loss will be greater if the part on the outer periphery side of the position 5 where the distance from the center is greater than 0.9 in terms of the ratio with respect to the radius of optical fiber preform is made of pure silica glass since the Rayleigh scattering intensity changes when the tension is made greater. Therefore, by using an optical fiber having a claddingportion 10 whose part extending to the outermost periphery from a position where the distance from the center of the optical fiber preform is within the range from 0.7 to 0.9 in terms of the ratio with respect to the radius of the optical fiber preform is made of highly pure silica glass, the Rayleigh 15 scattering intensity is restrained from increasing evenwhen a high tension acts on the optical fiber, whereby the increase in loss canbe suppressed. Here, thehighlypure silica glass refers to silica glass whose relative refractive index difference with respect to pure silica glass in a state 20 containing a dopant or the like is 0.001 or less. In the method of making an optical fiber in accordance with the present invention, a part of the optical fiber having a temperature higher than 1700 0 C before being coated with the resin may be cooled at a cooling rate of 4000 0 C/s or 25 higher. When the part of optical fiber having a temperature 13 FP0O-0085-00 higher than 1700*C is cooled at a cooling rate of 4000*C/s or higher, the height of equipment for carrying out drawing can be lowered. Since the structural relaxation of atoms proceeds in a very short period of time, the equilibrium 5 state at each temperature can be maintained even when cooled at a cooling rate of 4000*C/s or higher, whereby no influence is exerted upon the Rayleigh scattering intensity. In view of the above-mentioned results of studies, for achieving the above-mentioned object, the present invention 10 provides an apparatus for making an optical fiber for drawing an optical fiber preform upon heating and coating thus drawn optical fiber with a resin; the apparatus comprising a heating furnace, disposed between a drawing furnace for drawing the optical fiber preformupon heating and a resin coating section 15 for coating the drawn optical fiber with the resin, for heating the drawn optical fiber such that a segment of the optical fiber yielding a temperature difference of at least 50*C in a part of the optical fiber attaining a temperature of 1300 to 1700*C is cooled at a cooling rate of 1000 0 C/s or 20 less. Since the segment of optical fiber yielding a temperature difference of at least 50 0 C in the part of optical fiber before being coated with a resin where the temperature ranges from 1300 to 1700 0 C is slowly cooled at a cooling 25 rate of 1000 0 C/s or less by the heating furnace disposed between the drawing furnace and the resin coating section, 14 FPOO-0085-00 the optical fiber further lowers its fictive temperature, and the disorder in atomic arrangement is reduced, whereby it is possible to make an optical fiber having lowered its transmission loss by reducing the Rayleigh scattering 5 intensity within a very short period from the drawing upon heating to the resin coating. Also, since the Rayleigh scattering intensity is reduced by controlling the cooling rate of optical fiber after drawing before being coated with a resin, the heat treatment for reheating such as that in 10 the above-mentioned prior art is unnecessary, whereby the apparatus is quite easily applicable to the mass production of optical fiber whose surfaces are coated with a resin. The apparatus for making an optical fiber in accordance with the present invention may further comprise atmosphere 15 gas supplying means for supplying, as an atmosphere gas of the optical fiber within the heating furnace, an atmosphere gas having a thermal conductivity on a par with or lower than that of an atmosphere gas in the drawing furnace. When the atmosphere gas supplying means is further 20 provided, the atmosphere gas of optical fiber within the heating furnace lowers its thermal conductivity, so that the cooling rate within the heating furnace can be lowered, which enables the optical fiber to further lower its transmission loss. 25 The apparatus for making an optical fiber in accordance with the present invention may further comprise outside 15 FP00-0085-00 diameter measuring means for measuring the outside diameter of the optical fiber having exited from the heating furnace, and control means for controlling the drawing rate of the optical fiber according to a result of measurement by the 5 outside diameter measuring means such that the outside diameter of the optical fiber attains a predetermined value. When the control means is further provided, the outside diameter of the optical fiber in which the outer diameter length is in a stable state is measured, and the drawing 10 rateof optical fiberiscontrolledaccordingtothusmeasured outside diameter, whereby the drawing rate of optical fiber can appropriately be controlled. Brief Description of the Drawings Fig. 1 is a schematic diagram showing a first embodiment 15 of the apparatus and method for making an optical fiber in accordance with the present invention; Fig. 2 is a chart showing examples according to the first embodiment of the apparatus and method for making an optical fiber in accordance with the present invention, and 20 comparative examples; Fig. 3 is a schematic diagram showing a modified example of the first embodiment of the apparatus and method for making an optical fiber in accordance with the present invention; Fig. 4 is a schematic diagram showing a modified example 25 of the first embodiment of the apparatus and method for making an optical fiber in accordance with the present invention; 16 FPO0-0085-00 Fig. 5 is a schematic diagram showing a second embodiment of the apparatus and method for making an optical fiber in accordance with the present invention; Fig. 6 is a chart showing examples according to the 5 second embodiment the apparatus and method for making an optical fiber in accordance with the present invention, and comparative examples; Fig. 7A is a diagram showing the optical fiber preform used in an experimental example of the second embodiment 10 of the apparatus and method for making an optical fiber in accordance with the present invention; Fig. 7B is a chart showing the refractive index of the optical fiber preform used in the experimental example of the second embodiment of the apparatus and method for making 15 an optical fiber in accordance with the present invention; and Fig. 8 is a chart showing the relationship between the cooling rate and Rayleigh scattering coefficient of the optical fiber preform. 20 Best Modes for Carrying Out the Invention Embodiments of the present invention will be explained with reference to the drawings. In the explanation of the drawings, constituents identical to each other will be referred to with numerals or letters identical to each other 25 without repeating their overlapping descriptions. First Embodiment 17 FPOO-0085-00 To begin with, a first embodiment of the apparatus and method for making an optical fiber in accordance with the present invention will be explained. Adrawing apparatus 1 is an apparatus for drawing silica 5 type optical fiber; and comprises a drawing furnace 11, a heating furnace 21 for annealing, and a resin curing section 31, which are disposed in this order in the direction of drawing an optical fiber preform 2 (from the upper side to the lower side in Fig. 1) . The optical fiber preform 2 held 10 by a preform supplying apparatus (not depicted) is supplied to the drawing furnace 11, and the lower end of the optical fiber preform 2 is heated and softened by a heater 12 within the drawing furnace 11, so as to draw an optical fiber 3. An inert gas supply passage 15 from an inert gas supply section 15 14 is connected to a muffle tube 13 of the drawing furnace 11, so that an inert gas atmosphere is attained within the muffle tube 13 of drawing furnace 11. The optical fiber 3 drawn upon heating is rapidly cooled to about 1700 0 C by the inert gas within the muf fle tube 13. Thereafter, the optical 20 fiber 3 is taken out of the drawing furnace 11 from the lower part of the muffle tube 13, and is cooled with air between the drawing furnace 11 and the heating furnace 21. As the inert gas, N 2 gas can be used, for example. This N 2 gas has a thermal conductivitycoefficient A (T=300K) of 26mW/ (m-K) . 25 The thermal conductivity coefficient A (T = 300 K) of air is 26 mW/(m-K). 18 FP00-0085-00 The air-cooled optical fiber 3 is sent to the heating furnace 21, so that a predetermined segment of the optical fiber 3 is heated and then is annealed at a predetermined cooling rate. The heating furnace 21 has a muffle tube 23 5 through which the optical fiber 3 passes. The muffle tube 23 is set such that its total length. L2 (m) in the drawing direction of the optical fiber preform 2 (in the vertical direction of Fig. 1) satisfies: L2:-V/8 (3) 10 where V is the drawing rate (m/s) . Also, in the heating furnace 21, the muffle tube 23 is set at a position where the temperature (entering temperature) of the optical fiber immediately before entering the muffle tube 23 falls within the range from 1400 to 1800 0 C, so as to satisfy, with respect 15 to the drawing furnace 11, L1s0.2xV (4) where Li is the distance (m) from the lower end of heater 12 of drawing furnace 11 to the upper end 6f muffle tube 20 23; and V is the drawing rate (m/s) . The temperature of a heater 22 in the heating furnace 21 is set such that the temperature at the furnace center (the part through which the optical fiber 3 passes) attains a temperature within the range from 25 1200 to 1600*C, in particular within the range from 1300 to 1500 0 C. 19 FP00-0085-00 Upon the above-mentioned setting of position and length of heating furnace 21 (muf fle tube 23) , the segment of optical fiber 3 yielding a temperature difference of at least 50'C in the part of optical fiber 3 drawn upon heating where the 5 temperature ranges from 1200 to 1700*C in the heating furnace 21, e.g., the part where the temperature of optical fiber 3 ranges from 1400 to 1600'C (the segment yielding a temperature difference of 200*C) is annealed at a cooling rate of 1000*C/s or less. Here, when the temperature of 10 furnace center is set to a temperature within the range from 1300 to 1600 0 C, the section where the optical fiber 3 yields a temperature difference of at least 500C in the part of optical fiber 3 drawn upon heating where the temperature ranges from 1400 to 1600*C is annealed at a cooling rate 15 of 1000*C/s or less. Connected to the muffle tube 23 of heating furnace 21 is an N 2 gas supply passage 25 from an N 2 gas supply section 24, so that the muffle tube 23 of heating furnace 21 attains an N 2 gas atmosphere therewithin. In place of N 2 gas, gases 20 having a relatively large molecular weight such as air and Ar, and the like can be used as well. When a carbon heater is employed, it is necessary to use an inert gas as a matter of course. Of the optical fiber 3 having exited from the heating 25 furnace 21, the outside diameter is measured online by an outside diameter meter 41 acting as outside diameter 20 FPOO-0085-00 measuring means, and thus measured value is fed back to a driving motor 43 for driving a drum 42 to rotate, such that the outside diameter is controlled so as to become constant. The output signal from the outside diameter meter 41 is sent 5 toacontrolunit44actingascontrolmeans, wherethe rotating speed of drum 42 (driving motor 43) is determined by an arithmetic operation such that the outside diameter of optical fiber 3 attains a predetermined value which has been set beforehand. The output signal indicative of the rotating 10 speed of drum 42 (driving motor 43) determined by the arithmetic operation is outputted from the control unit 44 to a driving motor driver (not depicted) , whereas the driving motor driver controls the rotating speed of the driving motor 43 according to the output signal from the control unit 44. 15 Thereafter, the optical fiber 3 is coated with a UV resin 52 by a coating die 51, and the UV resin 52 is cured by a UV lamp 32 of the resin curing section 31, whereby a coated optical fiber 4 is obtained. By way of a guide roller 61, the coated optical fiber 4 is taken up by the drum 42. 20 The drum 42 is supported by a rotary driving shaft 45, whereas an end part of the rotary driving shaft 45 is connected to the driving motor 43. Here, the coating die 51 and the resin curing section 31 constitute the resin coating section in each claim. The resin coating section may be configured so 25 as to apply a thermosetting resin, which is to be cured by a heating furnace. 21 FP00-0085-00 Though the inert gas supply passage 15 from the inert gas supply section 14 is connected to the muffle tube 13 of drawing furnace 11 so that an inert gas atmosphere is attained within the muffle tube 13 of drawing furnace 11, 5 an N 2 gas supply section may be provided as the inert gas supply section 14 so as to supply N 2 gas into the muffle tube 13 such that an N 2 gas atmosphere is attained therein. When the drawing rate is low, e.g., 100 m/min, the optical fiber 3 may be cooled to about 1000 0 C within the drawing furnace 10 11 (muffle tube 13) in an He gas atmosphere. This is the reason why N 2 gas is supplied into the muffle tube 13, so as to attain an N 2 gas atmosphere within the muffle tube 13, thereby causing the optical fiber 3 to attain a temperature of about 17000C at the exit of drawing furnace 11 (muffle 15 tube 13). As a matter of course, He gas supply section and

N

2 gas supply section may be provided, so as to supply He gas and/or N 2 gas into the muffle tube 13 in response to the drawing rate. Next, with reference to Fig. 2, results of experiments 20 according to the apparatus and method for making an optical fiber in accordance with the first embodiment using the above-mentioned drawing apparatus 1 will be explained. The conditions common in these experiments are as follows. As the optical fiber preform 2, one having an outside diameter 25 of 35 mm was used, and an optical fiber 3 having an outside diameter of 125 pm was drawn from the optical fiber preform 22 FP00-0085-00 2. The temperature of drawing furnace was about 2000'C at the surface of the inner periphery of the muffle tube. In the following Examples 1 to 8 and Comparative Examples 1 to 4, the surface temperature of optical fiber 3 is taken 5 as the temperature of optical fiber 3. The dif ference between the surf ace temperature of optical fiber 3 and the temperature within the optical fiber 3 is on the order of 20 to 100*C. The surface temperatures of the inner peripheral faces (surfaces opposing the surfaces of optical fiber preform 10 2 and optical fiber 3) of muffle tubes 13, 23 are taken as the temperatures of drawing furnace 11 and heating furnace 21, respectively. In each of Examples 1 to 8 and Comparative Examples 1 to 4, N 2 gas was used as the inert gas. Examples 1 to 4 are examples according to the apparatus 15 and method for making an optical fiber in accordance with the above-mentioned first embodiment, whereas Comparative Examples 1 and 2 are comparative examples carried out for comparison with the examples according to the apparatus and method for making an optical fiber in accordance with the 20 above-mentioned first embodiment. Example 1 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30 mm) in which Li = 0.4 m and L2 = 0.5 m, an optical fiber was drawn. The optical 25 fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped 23 FP0O-0085-00 glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace center) were set to 4 m/s, 0.196 N (20 gf), and 1300'C, respectively. In this case, the temperature of the optical 5 fiber immediately before entering the heating furnace (entering temperature) was 1600'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1350*C in terms of the surface temperature of 10 optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1600 to 1350 " C in the drawn optical fiber is assumed to have been cooled at an average annealing rate of about 2000'C/s in a segment of 0.5 m which is the total length of heating furnace. 15 As for the position of muffle tube, 0.4 < 0.8 (= 4 x 0.2), thereby satisfyingthe above-mentioned expression (4) As for the total length of muffle tube, 0.5 = 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3) . The transmission loss of drawn optical fiber (transmission loss 20 with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.167 dB/km. Example 2 Using a heating furnace having a muf fle tube (with an inner peripheral diameter of about 30 mm) in which Li = 0.4 25 m and L2 = 1.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure 24 FPOO-0085-00 silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace center) were set to 4 m/s, 0.196 N (20 gf), and 1300*C, 5 respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1600'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating 10 furnace was 1350'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1600 to 1350 * C in the drawn optical fiber is assumed to have been cooled at an average annealing rate of about 1000'C/s in a segment of 1.0 m which is the 15 total length of heating furnace. As for the position of muffle tube, 0.4 < 0.8 (= 4 x 0.2), thereby satisfying the above-mentioned expression (4) . As for the total length of muffle tube, 1.0 > 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3). The 20 transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.165 dB/km. Example 3 Using a heating furnace having a muf fle tube (with an 25 inner peripheral diameter of about 30 mm) in which Li = 0.4 m and L2 = 2.0 m, an optical fiber was drawn. The optical 25 FPOO-0085-00 fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace 5 center) were set to 4 m/s, 0.196 N (20 gf), and 1300'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1600"C in terms of the surface temperature of optical fiber, whereas the temperature of 10 the optical fiber immediately after exiting from the heating furnace was 1300'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1600 to 1300*C in the drawn optical fiber is assumed to have been cooled at an average annealing 15 rate of about 600*C/s in a segment of 2.0 m which is the total length of heating furnace. As for the position of muffle tube, 0.4 < 0.8 (= 4 x 0.2), therebysatisfyingthe above-mentionedexpression (4). As for the total length of muffle tube, 2.0 > 0.5 (= 4/8), 20 thereby satisfying the above-mentioned expression (3). The transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.164 dB/km. Example 4 25 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30 mm) in which L1 = 0.6 26 FPOO-0085-00 m and L2 = 1.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the 5 temperature of heating furnace (temperature at the furnace center) were set to 4 m/s, 0.196 N (20 gf), and 1300'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1400'C in terms of the surface 10 temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1300'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1400 to 1300 * C -in the drawn optical 15 fiber is assumed to have been cooled at an average annealing rate of about 250'C/s in a segment of 1.0 m which is the total length of heating furnace. As for the position of muffle tube, 0.8 = 0.8 (= 4 x 0.2), thereby satisfyingthe above-mentionedexpression (4). 20 As for the total length of muffle tube, 1.0 > 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3). The transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.167 dB/km. 25 Comparative Example 1 An optical fiber was drawn in a state where the heating 27 FP00-0085-00 furnace was removed. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate and the drawing tension were set to 2 to 10 m/s and 0.196 N (20 5 gf), respectively. In this case, the part of optical fiber attaining a temperature of 1300 to 1700'C was cooled at an annealing rate of about 5000*C/s. The transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength 10 of 1.55 pam) was measured and found to be 0.168 dB/km, and there was no dependence on the drawing rate. Comparative Example 2 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30 mm) in which Li = 1.0 15 m and L2 = 1.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace 20 center) were set to 4 m/s, 0.196 N (20 gf), and 1300*C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1000*C in terms of the surface temperature of optical fiber. 25 As for the position of muffle tube, 1.2 > 0.8 (= 4 x 0.2), thereby failing to satisfy the above-mentioned 28 FPO0-0085-00 expression (4) . As for the total length of muffle tube, 1.0 > 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3) . The transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength 5 of 1.55 pm) was measured and found to be 0.168 dB/km as with the transmission loss in Comparative Example 1 in which the heating furnace was removed. As in the foregoing, it has been verified that the transmission loss with respect to light having a wavelength 10 of 1.55 pm is 0.164 to 0.167 dB/km in Examples 1 to 4, and thus can be lowered by the range of 0.001 to 0.004 dB/km from the transmission loss of 0.168 dB with respect to light having a wavelength of 1.55 pm in Comparative Examples 1 and 2. If Li as the position of muf fle tube of heating furnace 15 is made greater than 0.8 m (so as to be distanced from the drawing furnace) when the drawing rate is 4 m/s, it will be harder to heat the part of optical fiber attaining a temperature of 1200 to 1700'C after drawing, so that the cooling rate in this part cannot be lowered, whereby the 20 transmission loss will increase. Also, even when the muffle tube of heating furnace is disposed at a position satisfying expression (4) , if the total length of muffle tube is shorter than 0.5 m, it will be harder to heat the part of drawn optical fiber attaining a temperature of 1200 to 1700'C, so that 25 the cooling rate in this part cannot be lowered, whereby the transmission loss will increase. 29 FP00-0085-00 Next, experiments were carried out while changing temperature conditions of the heating furnace (surface temperature of inner peripheral face of muffle tube). Examples 5 and 6 are examples according to the apparatus 5 and method for making an optical fiber in accordance with the above-mentioned first embodiment, whereas Comparative Example 3 is a comparative example carried out for comparison with the examples according to the apparatus and method for making an optical fiber in accordance with the 10 above-mentioned first embodiment. Example 5 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30 mm) in which Li = 0.4 m and L2 = 1.0 m, an optical fiber was drawn. The optical 15 fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace center) were set to 4 m/s, 0.196 N (20 gf), and 1500'C, 20 respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1600'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating 25 furnace was 1530'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part 30 FPOO-0085-00 attaining a temperature of 1600 to 1530'C in the drawn optical fiber is assumed to have been cooled at an average annealing rate of about 280*C/s in a segment of 1.0 m which is the total length of heating furnace. 5 As for the position of muffle tube, 0.4 < 0.8 (= 4 x 0.2), thereby satisfyingthe above-mentionedexpression (4). As for the total length of muffle tube, 1.0 > 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3). The transmission loss of drawn optical fiber (transmission loss 10 with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.162 dB/km. Example 6 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30 mm) in which Ll = 0.4 15 m and L2 = 1.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace 20 center) were set to 4 m/s, 0.196 N (20 gf), and 1200'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1600'C in terms of the surface temperature of optical fiber, whereas the temperature of 25 the optical fiber immediately after exiting from the heating furnace was 1250'C in terms of the surface temperature of 31 FPO0-0085-00 optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1600 to 1250'C in the drawn optical fiber is assumed to have been cooled at an average annealing rate of about 350*C/s in a segment of 1.0 m which is the 5 total length of heating furnace. As for the position of muffle tube, 0.4 < 0.8 (= 4 x 0.2), therebysatisfyingthe above-mentionedexpression (4) As for the total length of muffle tube, 1.0 > 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3). The 10 transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.167 dB/km. Comparative Example 3 Using a heating furnace having a muf fle tube (with an 15 inner peripheral diameter of about 30 mm) in which Ll = 0. 4 m and L2 = 1.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the 20 temperature of heating furnace (temperature at the furnace center) were set to 4 m/s, 0.196 N (20 gf), and 1000*C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 16.00*C in terms of the surface 25 temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating 32 FP00-0085-00 furnace was 1050'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1600 to 1050 * C in the drawn optical fiber is assumed to have been cooled at an average annealing 5 rate of about 2200'C/s in a segment of 1.0 m which is the total length of heating furnace. In Comparative Example 3, as for the position of muffle tube, 0.4 < 0.8 (= 4 x 0.2), thereby satisfying the above-mentioned expression (4) . As for the total length of 10 muffle tube, 1.0 > 0.5 (= 4/8), thereby satisfying the above-mentioned expression (3). However, the optical fiber failed to attain a temperature of 1200*C or higher in the part of heating furnace. The transmission loss of drawn optical fiber (transmission loss with respect to light having 15 a wavelength of 1.55 pm) was measured and found to be 0.168 dB/km as with the transmission loss in Comparative Example 1 in which the heating furnace was removed. As in the foregoing, it has been verified that the transmission loss with respect to light having a wavelength 20 of 1.55 pm is 0.162 to 0.167 dB/km in Examples 5 to 6, and thus can be lowered by the range of 0.001 to 0.006 dB/km from the transmission loss of 0.168 dB with respect to light having a wavelength of 1.55 pm in Comparative Example 3. When the temperature of heating furnace (surface temperature 25 of inner peripheral face of muffle tube) is 1200*C or higher, as can be seen from the results of experiments, the part 33 FP0O-0085-00 of optical fiber attaining a temperature of 1300 to 1700'C after drawing can be heated, so that the cooling rate in this part is lowered, whereby the transmission can be reduced. In particular, as can be seen from Examples 2 and 5, the 5 transmission loss can further be reduced when the temperature of the heating furnace (surface temperature of inner peripheral face of muffle tube) is 1300 to 1500'C. Next, experiments were carried out while changing drawing rate conditions. Examples 7 and 8 are examples 10 according to the apparatus and method for making an optical fiber in accordance with the above-mentioned first embodiment, whereas Comparative Example 4 is a comparative example carried out for comparison with the examples according to the .apparatus and method for making an optical fiber in 15 accordance with the above-mentioned first embodiment. Example 7 Using a heating furnace having a muf fle tube (with an inner peripheral diameter of about 30 mm) in which Ll = 0.8 m and L2 = 1.0 m, an optical fiber was drawn. The optical 20 fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace center) were set to 8 m/s, 0.196 N (20 gf), and 1300'C, 25 respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace 34 FPOO-0085-00 (entering temperature) was 1700*C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1550"C in terms of the surface temperature of 5 optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1700 to 15500 C in the drawn optical fiber is assumed to have been cooled at an average annealing rate of about 1200*C/s in a segment of 1.0 m which is the total length of heating furnace. 10 As for the position of muffle tube, 0.8 < 1.6 (= 8 x 0.2), thereby satisfying the above-mentioned expression (4) . As for the total length of muffle tube, 1.0 = 1.0 (= 8/8), thereby satisfying the above-mentioned expression (3) . The transmission loss of drawn optical fiber (transmission loss 15 with respect to light.having a wavelength of 1.55 pm) was measured and found to be 0.167 dB/km. Example 8 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30 mm) in which Li = 0.8 20 m and L2 = 2.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (temperature at the furnace 25 center) were set to 8 m/s, 0.196 N (20 gf), and 1300'C, respectively. In this case, the temperature of the optical 35 FPOO-0085-00 fiber immediately before entering the heating furnace (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating 5 furnace was 1450'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1700 to 1450'C in the drawn optical fiber is assumed to have been cooled at an average annealing rate of about 1000*C/s in a segment of 2.0 m which is the 10 total length of heating furnace. As for the position of muffle tube, 0.8 < 1.6 (= 8 x 0.2) , thereby satisfying the above-mentioned expression (4) . As for the total length of muffle tube, 2.0 > 1.0 (= 8/8), thereby satisfying the above-mentioned expression (3). The 15 transmission loss of drawn optical fiber (transmission loss with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.165 dB/km. Comparative Example 4 Using a heating furnace having a muffle tube (with an 20 inner peripheral diameter of about 30 mm) in which Ll = 2.0 m and L2 = 1.0 m, an optical fiber was drawn. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the 25 temperature of heating furnace (temperature at the furnace center) were set to 8 m/s, 0.196 N (20 gf), and 1300'C, 36 FP00-0085-00 respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1000'C in terms of the surface temperature of optical fiber. 5 As for the position of muffle tube, 2.0 > 1.6 (= 8 x 0.2), thereby failing to satisfy the above-mentioned expression (4) . As for the total length of muffle tube, 1.0 = 1.0 (= 8/8), thereby satisfying the above-mentioned expression (3) . The transmission loss of drawn optical fiber 10 (transmission loss with respect to light having a wavelength of 1.55 pm) was measured and found to be 0.168 dB/km as with the transmission loss in Comparative Example 1 in which the heating furnace was removed. As in the foregoing, it has been verified that the 15 transmission loss with respect to light having a wavelength of 1.55 pm is 0.165 to 0.167 dB/km in Examples 7 and 8, and thus can be lowered by the range of 0.001 to 0.003 dB/km from the transmission loss of 0.168 dB with respect to light having a wavelength of 1.55 pm in Comparative Example 4. 20 If Ll as the position of muffle tube of heating furnace is 2.0 m when the drawing rate is 8 m/s, it will be harder to heat the part of optical fiber attaining a temperature of 1200 to 1700'C after drawing, so that the cooling rate in this part cannot be lowered, whereby the transmission loss 25 will increase. Also, even when the muffle tube of heating furnace is disposed at a position satisfying expression (4), 37 FPOO-0085-00 if the total length of muffle tube is shorter than 1.0 m, it will be harder to heat the part of drawn optical fiber attaining a temperature of 1200 to 1700'C, so that the cooling rate in this part cannot be lowered, whereby the transmission 5 loss will increase. As can be seen from the results of experiments mentioned above, since the heating furnace 21 in which the optical fiber 3 drawn upon heating by the drawing furnace 11 before being coated with the UV resin 52 is heated at a temperature 10 of 1200 to 1700'C is disposed between the drawing furnace 11andresincuringsection31 (coatingdie51) intheapparatus and method for making an optical fiber in accordance with the first embodiment, cooling rate is lowered in a predetermined segment in the part attaining a temperature 15 ranging from 1200 to 1700'C in the above-mentioned optical fiber 3, so that the fictive temperature of optical fiber 3 decreases, thus lowering the disorder in atomic arrangement, whereby it becomes possible to make the optical fiber 3 having decreased its transmission loss by reducing its Rayleigh 20 scattering intensity during the period from the drawing upon heating to the coating with the UV resin 52. Also, since the Rayleigh scattering intensity is reduced by controlling the cooling rate of optical fiber 3 after drawing before being coated with the UV resin 52, the heat treatment for 25 reheating such as that in the above-mentioned prior art is unnecessary, wherebythe apparatus is quite easilyapplicable 38 FP00-0085-00 to the mass production of the coated optical fiber 4 whose surface is coated with the UV resin 52 cured thereon. When the optical fiber 3 drawn upon heating by the drawing furnace 11 before being coated with the UV resin 5 52 is heated at a temperature of 1300 to 1600 0 C in the heating furnace 21, the cooling rate of optical fiber 3 decreases at a predetermined segment in the part of optical fiber 3 attaining a temperature of 1200 to 1700 0 C, so that the optical fiber 3 lowers its fictive temperature, whereby its Rayleigh 10 scattering intensity can further be reduced. When the position of muffle tube 23 of heating furnace 21 is located at a position satisfying the above-mentioned expression (4), a predetermined segment in the part attaining a temperature of 1200 to 1700 0 C in the optical fiber 3 after 15 being drawn upon heating in the drawing furnace 11 before being coated with the UV resin 52 can be heated securely, whereby the cooling rate in this part can be lowered appropriately. When the position of muffle tube 23 of heating furnace 20 21 is located at a position where the temperature (entering temperature) of optical fiber immediately before entering the muffle tube 23 ranges from 1400 to 1800 0 C, apredetermined segment in the part attaining a temperature of 1200 to 1700 0 C in the optical fiber 3 after being drawn upon heating in 25 the drawing furnace 11 before being coated with the UV resin 52 can be heated securely, whereby the cooling rate in this 39 FP0O-0085-00 part can be lowered appropriately. When the total length of muffle tube 23 of heating furnace 21 satisfies the above-mentioned expression (3), a predetermined segment in the part attaining a temperature 5 of 1200 to 1700 0 C in the optical fiber 3 after being drawn upon heating in the drawing furnace 11 before being coated with the UV resin 52 can be heated securely, whereby the cooling rate in this part can be lowered appropriately. Since anN 2 gas atmosphere is attained within the muffle 10 tube 23 of heating furnace 21, the cooling rate within the heating furnace 21 (muffle tube 23) can be lowered, whereby the optical fiber 3 can further lower its transmission loss. When an He gas atmosphere is attained within the muffle tube 13 of drawing furnace 11, on the other hand, the cooling 15 rate of optical fiber 3 within the drawing furnace (muffle tube 13) becomes about 30000*C/s. In this case, since air-cooling is carried out between the drawing furnace 11 and heating furnace 21, the cooling rate of optical fiber 3becomes 4000 to 5000*C/s, so that the optical fiberpreform 20 2 is softened upon heating and is rapidly cooled until it tapers down to a predetermined diameter, whereby the optical fiber 3 can be restrained from fluctuating its outside diameter. When the He gas atmosphere is attained within the muffle tube 13 of drawing furnace 11 while air-cooling is 25 carried out between the drawing furnace 11 and heating furnace 21, the part of optical fiber 3 yielding a temperature higher 40 FP00-0085-00 than 1700 0 C before entering the heating furnace 21 is cooled at a cooling rate of 4000*C/s or higher, whereby the height of equipment needed for cooling the optical fiber 3 can be reduced. At a temperature higher than 1700 0 C, the fictive 5 temperature will be lower than 1700*Cevenwhen rapidly cooled at a rate of about 30000*C/s, for example, thus exerting no influences on Rayleigh scattering. Since the outside diameter meter 41 for measuring the outside diameter of optical fiber 3 having exited from the 10 heating furnace 21 and the control unit 44 for controlling the rotating speed of drum 42 (driving motor 43) according to the output signal from the outside diameter meter 41 such that the outside diameter of optical fiber 3 attains a predetermined value are provided, the outside diameter of 15 the optical fiber 3 having exited from the heating furnace 21 with an outside diameter length in a stable state is measured, and the rotating speed of the drum 42 (driving motor 43) is controlled according to this stable outside diameter, whereby the drawing rate of optical fiber 3 can be controlled 20 appropriately. Next, with reference to Figs. 3and4, amodified example of the above-mentioned first embodiment will be explained. As shown in Fig. 3, in a drawing apparatus 101 for a silica type optical fiber, the heater 22 of heating furnace 21 25 includes a first heater 71, a second heater 72, and a third heater 73. The heaters 71, 72, 73 are successively disposed 41 FPOO-0085-00 in this order in the direction of drawing the optical fiber preform 2 (from the upper side to lower side in Fig. 2). The respective temperatures of heaters 71, 72, 73 are regulated so as to satisfy: 5 T1=T2+25*C (5) T3=T2-25 0 C (6) where Tl is the surface temperature of inner peripheral face of muffle tube 23 at a position corresponding to the first 10 heater 71; T2 is the surface temperature of inner peripheral face of muffle tube 23 at a position corresponding to the second heater 72; and T3 is the surface temperature of inner peripheral face 15 of muffle tube 23 at a position corresponding to the third heater 73. Here, the difference between Tl and T2 or the difference between T2 and T3 is not limited to 25*C mentioned above, and the temperature difference may be about 30 0 C, for example. 20 Thus, a temperature gradient yielding higher and lower temperatures on the drawing furnace 11 side and the resin coating section 31 (coating die 51) side, respectively, is provided within the muffle tube 23 of heating furnace 21 when the first heater 71, second heater 72, and third heater 25 73 are provided. The optical fiber 3 drawn upon heating in the drawing furnace 11 has such a temperature distribution 42 FP00-0085-00 that its temperature lowers from the drawing furnace 11 side to the resin coating section 31 (coatingdie 51) side. Hence, when the first heater 71, second heater 72, and third heater 73 having regulated their temperatures are disposed as 5 mentioned above, the heating furnace 21 is provided with such a temperature gradient that higher and lower temperatures are attained on the drawing furnace 11 side and the resin coating section 31 (coating die 51) side, respectively, so that the muffle tube 23 has a temperature 10 distribution corresponding to the temperature of optical fiber 3 therewithin and appropriately keeps its temperature difference from the optical fiber 3, whereby the optical fiber 3 can be cooled at a further appropriate cooling rate. As another modified example, the heating furnace 21 15 may be provided integrally with the drawing furnace 11 so as to be continuous therewith as in the drawing apparatus 201 shown in Fig. 4. When the heating furnace 21 is thus provided integrally with the drawing furnace 11 so as to be continuous therewith, the cooling rate decreases in a 20 predetermined segment in the part yielding a temperature of 1200 to 1700 0 C in the optical fiber 3 after drawing upon heating in the drawing furnace 11 before being coated with the UV resin 52, so that the optical fiber 3 lowers its fictive temperature, and the disorder in atomic arrangement is 25 reduced, whereby it is possible to make the optical fiber 3 having lowered its transmission loss by reducing the 43 FP00-0085-00 Rayleigh scattering intensity within a very short period from the drawing upon heating to the coating with the UV resin 52. Second Embodiment 5 Next, with reference to Fig. 5, a second embodiment of the apparatus and method for making an optical fiber in accordance with the present invention will be explained. A drawing apparatus 301 is a drawing apparatus for a silica type optical fiber; and comprises a drawing furnace 10 11, a heating furnace 21 for annealing, and a resin curing section 31, which are disposed in this order in the direction of drawing an optical fiber preform 2 (from the upper side to the lower side in Fig. 5). The optical fiber preform 2 held by a preform supplying apparatus (not depicted) is 15 supplied to the drawing furnace 11, and the lower end of the optical fiber preform 2 is heated and softened by a heater 12 within the drawing furnace 11, so as to draw an optical fiber 3. He gas/N 2 gas supply passage 315 for selectively supplying He gas or N 2 gas from He gas/N 2 gas supply section 20 314 is connected to a muffle tube 13 of the drawing furnace 11, so that anHe gas atmosphereorN2 gas atmosphere is attained within the muffle tube 13 of drawing furnace 11. The optical fiber 3 drawn upon heating is rapidly cooled to about 1700'C within the muffle tube 13. Thereafter, the optical fiber 25 3 is taken out of the drawing furnace 11 from the lower part of the muffle tube 13, and is cooled with air between the 44 FP00-0085-00 drawing furnace 11 and the heating furnace 21. He gas has a thermal conductivity coe f ficientA (T=300K) of 150mW/ (m-K),

N

2 gas has a thermal conductivity coefficient A (T = 300 K) of 26mW/ (m-K) , and air has a thermal conductivity coefficient 5 X (T = 300 K) of 26 mW/ (m-K) . The air-cooled optical fiber 3 is sent to the heating furnace 21, so that a predetermined segment of optical fiber 3 is heated and then is annealed at a predetermined cooling rate. The annealing in the heating furnace 21 is carried 10 out by cooling at a cooling rate of 1000*C/s or less the segment where the optical fiber 3 yields a temperature dif ference of at least 50*C in the part attaining a temperature of 1300 to 1700*C in the optical fiber 3 drawn upon heating. In particular, it is preferred that the segment where the 15 optical fiber 3 yields a temperature difference of at least 50*C in the part attaining a temperature of 1400 to 1600*C in the optical fiber 3 drawn upon heating be cooled at a cooling rate of 1000 0 C/s or less. Therefore, the installing positions of the heater 22 and muffle tube 23 of heating 20 furnace 21 and the total length thereof in the drawing direction of optical fiber preform 2 (the vertical direction in Fig. 5) are set in view of the drawing rate. Here, the drawing rate is needed to be taken into consideration, since the position at which the optical fiber 3 attains a given 25 temperature descends as the drawing rate increases. Also, the temperature of heater 22 of heating furnace 21 is set 45 FP0O-0085-00 such that the segment where the optical fiber 3 located within the muffle tube 23 attains a temperature difference of at least 50*C is cooled at a cooling rate of 1000*C/s or less. Connected to the muffle tube 23 of heating furnace 21 5 is an N 2 gas supply passage 25 from an N 2 gas supply section 24, so that the muffle tube 23 of heating furnace 21 attains an N 2 gas atmosphere therewithin. N 2 gas has a thermal conductivity lower than that of He gas, thereby acting to lower the cooling rate of optical fiber. In place of N 2 gas, 10 gases having a relatively large molecular weight such as air and Ar, and the like can be used as well. When a carbon heater is employed, it is necessary to use an inert gas as a matter of course. Next, with reference to Fig. 6, results of experiments 15 according to the apparatus and method for making an optical fiber in accordance with the second embodiment using the above-mentioned drawing apparatus 301 will be explained. The conditions common in these experiments are as follows. As the optical fiberpreform2, one having an outside diameter 20 of 35 mm was used, and an optical fiber 3 having an outside diameter of 125 pm was drawn from the optical fiber preform 2. The temperature of drawing furnace was about 2000*C (though slightly changed according to the drawing tension) at the surface of the inner periphery of the muffle tube. 25 In the following experimental examples (Examples 9 to 24), the surface temperature of optical fiber 3 is taken as the 46 FP00-0085-00 temperature of optical fiber 3. The difference between the surface temperature of optical fiber 3 and the temperature within the optical fiber 3 is on the order of 20 to 100'C. The surface temperatures of the inner peripheral faces 5 (surfaces opposing the surfaces of optical fiber preform 2 and optical fiber 3) of muffle tubes 13, 23 are taken as the temperatures of drawing furnace 11 and heating furnace 21, respectively. Examples 9 to 12 are examples according to the apparatus 10 and method for making an optical fiber in accordance with the above-mentioned second embodiment, whereas Comparative Examples 5 to 7 are comparative examples carried out for comparison with the examples according to the apparatus and method for making an optical fiber in accordance with the 15 above-mentioned second embodiment. Example 9 Using a heating furnace having a muffle tube with a total length (L2) of 2 m in the drawing direction of optical fiber preform (and an inner peripheral diameter of about 20 30 mm) , an optical fiber was drawn. Supplied into the drawing furnace (muffle tube) was N 2 gas. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of 25 heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.196 N (20 gf), 47 FP00-0085-00 and 1400'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature 5 of the optical fiber immediately after exiting from the heating furnace was 1450 * C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1450 to 1700 *C in the drawn optical fiber is assumed to have been cooled at an average cooling 10 rate of about 250 'C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.165 dB/km. The Rayleigh 15 scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0. 825 dBPm 4 /km. Example 10 Using a heating furnace having a muffle tube with a 20 total length (L2) of 2 m in the drawing direction of optical fiber preform (and an inner peripheral diameter of about 30 mm) , an optical fiber was drawn. Supplied into the drawing furnace (muffle tube) was He gas. The optical fiber preform to be drawn had a core portion made of pure silica glass 25 and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of 48 FPOO-0085-00 heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 400 m/min, 0.294 N (30 gf), and 1200*C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating 5 furnace (entering temperature) was 1550'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1300 *C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part 10 attaining a temperature of 1300 to 1550 * C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 1000'C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect 15 to light having a wavelength of 1.55 pm) of the drawn optical fiberwas measuredand found tobe 0.167 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.838 dBpm 4 /km. 20 Example 11 Using a heating furnace having a muffle tube with a total length (L2) of 0 .5 m in the drawing direction of optical fiber preform (and an inner peripheral diameter of about 30 mm) , an optical fiber was drawn. - Supplied into the drawing 25 furnace (muffle tube) was N 2 gas. The optical fiber preform to be drawn had a core portion made of pure silica glass 49 FPOO-0085-00 and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.245 N (25 gf), 5 and 1450 "C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1550'C in terms of the surf ace temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the 10 heating furnace was 1500'C in terms of the surf ace temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1500 to 1550O Cin the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250'C/s in a segment of 0.5 m which is the 15 total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1. 55 pm) of the drawn optical fiber was measured and found to be 0.166 dB/km. The Rayleigh scattering coefficient determined from the measured data 20 of wavelength characteristic of transmission loss was 0.838 dBpam 4 /km. Example 12 Using a heating furnace having a muffle tube with a total length (L2) of 2 m in the drawing direction of optical 25 fiber preform (and an inner peripheral diameter of about 30 mm) , an optical fiber was drawn. Supplied into the drawing 50 FP00-0085-00 furnace (muffle tube) was N 2 gas. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of 5 heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 30 m/min, 0.196 N (20 gf), and 1400'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1700'C in terms of the 10 surf ace temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 142 0 ' C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1420 to 1700'C in the drawn optical 15 fiber is assumed to have been cooled at an average cooling rate of about 90'C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical 20 fiber was measured and found to be 0.160 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.805 dBipm 4 /km. Comparative Example 5 25 An optical fiber was drawn in a state where the heating furnace was removed. Supplied into the drawing furnace 51 FP00-0085-00 (muffle tube) was He gas. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate and the drawing tension were set to 100 m/min and 0.294 5 N (30 gf), respectively. In this case, the part attaining a temperature of 1300 to 1700'C in the optical fiber was cooled at an average cooling rate of about 30000'C/s. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical 10 fiber was measured and found to be 0.175 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.88 dBym 4 /km. Comparative Example 6 15 An optical fiber was drawn in a state where the heating furnace was removed. Supplied into the drawing furnace (muffle tube) was He gas. The optical fiber preform to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing 20 rate and the drawing tension were set to 100 m/min and 0.196 N (20 gf), respectively. In this case, the part attaining a temperature of 1300 to 1700'C in the optical fiber was cooled at an average cooling rate of about 5000'C/s. The transmission loss (transmission loss with respect 25 to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.170 dB/km. The Rayleigh 52 FPO0-0085-00 scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.85 dByPm 4 /km. Comparative Example 7 5 Using a heating furnace having a muffle tube with a total length (L2) of 2 m in the drawing direction of optical fiber preform (and an inner peripheral diameter of about 30 mm) , an optical fiber was drawn. Supplied into the drawing furnace (muffle tube) was N 2 gas. The optical fiber preform 10 to be drawn had a core portion made of pure silica glass and a cladding portion made of fluorine-doped glass. The drawing rate, the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.294 N (30 gf), 15 and 900'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1300'C in terms of the surf ace temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the 20 heating furnace was 1000'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1000 to 1300*C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250 *C/s in a segment of 2 m which is the total 25 length of heating furnace. The transmission loss (transmission loss with respect 53 FPOO-0085-00 to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found tobe 0.170 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.85 5 dBpm 4 /km. As in the foregoing, Examples 9 to 12 exhibited a Rayleigh scattering coefficient of 0.805 to 0.838 dBpm 4 /km and a transmission loss of 0.160 to 0.167 dB/km with respect to light having a wavelength of 1.55 pm, thus being able 10 to lower the Rayleigh scattering coefficient and reduce the transmission loss as compared with Comparative Examples 5 to 7 yielding a Rayleigh scattering coefficient of 0.85 to 0 .88 dBpm 4 /km and a transmission loss of 0.170 to 0.175 dB/km with respect to light having a wavelength of 1.55 pm. 15 Next, experiments were carried out under the experimental conditions of Example 9 mentioned above while changing the concentration of hydroxyl group contained in the core portion of optical fiber preform. Examples 13 and 14 are examples of the apparatus and method for making an 20 optical fiber in accordance with the above-mentioned second embodiment. Example 13 The core portion of the optical fiber preform to be drawn was caused to contain hydroxyl group such that the 25 transmission loss by hydroxyl group absorption at a wavelength of 1.38 pm became 0.02 dB/km. Supplied into the 54 FPOO-0085-00 drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 mm) was used, whereas the drawing rate, the drawing tension, and the temperature 5 of heating furnace (surface temperature at the inner -peripheral face of muffle tube) were set to 100 m/min, 0.196 N (20 gf), and 1400'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1700'C in 10 terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1450'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1450 to 1700'C 15 in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250'C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pim) of the drawn optical 20 fiber was measured and found to be 0.164 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.82 dBpam 4 /km. Example 14 25 The core portion of the optical fiber preform to be drawn was caused to contain hydroxyl group such that the 55 FP00-0085-00 transmission loss by hydroxyl group absorption at a wavelength of 1.38 pm became 0.5 dB/km. Supplied into the drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and 5 an inner peripheral diameter of about 30 mm) was used, whereas the drawing rate, the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.196 N (20 gf), and 1400'C, respectively. In this case, the 10 temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1450'C in terms of the 15 surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1450 to 1700'C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250'C/s in a segment of 2 m which is the total length of heating furnace. 20 The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.165 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0. 815 25 dB pm 4 /km. As in the foregoing, Examples 13 and 14 exhibited a 56 FPOO-0085-00 Rayleigh scattering coefficient of 0.815 to 0.82 dBpim 4 /km and a transmission loss of 0.164 to 0.165 dB/km with respect to light having a wavelength of 1.55 pm, thus being able to lower the Rayleigh scattering coefficient and reduce the 5 transmission loss as compared with Example 9 yielding a Rayleigh scattering coefficient of 0.825 dBpm 4 /km and a transmission loss of 0.165 dB/km with respect to light having a wavelength of 1. 55 pam. Though Example 14, in which hydroxyl group is contained in the core portion of the optical fiber 10 preform to be drawn such that the transmission loss caused by hydroxyl group absorption at a wavelength of 1.38 pm is 0.5 dB/km, yields a Rayleigh scattering coefficient lower than that in Example 9, the transmission loss caused by hydroxyl group absorption therein is at a level which cannot 15 be neglected, thus canceling out the effect of lowering the Rayleigh scattering coefficient, whereby transmission loss will be enhanced if hydroxyl group is contained more. Next, experiments were carried out under the experimental conditions of Example 9 mentioned above while 20 changing the concentration of Cl contained in the core portion of optical fiber preform. Examples 15 to 17 are examples of the apparatus and method for making an optical fiber in accordance with the above-mentioned second embodiment. Example 15 25 The core portion of the optical fiber preform to be drawn was caused to contain Cl such that the relative 57 FP00-0085-00 refractive index difference with respect to pure silica glass became 0.0001. Supplied into the drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral 5 diameter of about 30 mm) was used, whereas the drawing rate, the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.196 N (20 gf), and 1400'C, respectively. In this case, the temperature of the optical 10 fiber immediately before entering the heating furnace (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1450*C in terms of the surface temperature of 15 optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1450 to 1700 * C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250*C/s in a segment of 2 m which is the total length of heating furnace. 20 The transmission loss (transmission loss with respect to light having a wavelength of 1. 55 pm) of the drawn optical fiber was measured and found to be 0.164 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.82 25 dBpm 4 /km. Example 16 58 FP00-0085-00 The core portion of the optical fiber preform to be drawn was caused to contain Cl such that the relative refractive index difference with respect to pure silica glass became 0.0005. Supplied into the drawing furnace (muffle 5 tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 mm) was used, whereas the drawing rate, the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle 10 tube) were set to 100 m/min, 0.196 N (20 gf), and 1400'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of 15 the optical fiber immediately after exiting from the heating furnace was 1450"C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1450 to 1700 * C in the drawn optical fiber is assumed to have been cooled at an average cooling 20 rate of about 250 C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.163 dB/km. The Rayleigh 25 scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.815 59 FP00-0085-00 dBym 4 /km. Example 17 The core portion of the optical fiber preform to be drawn was caused to contain Cl such that the relative 5 refractive index difference with respect topure silica glass became 0.001. Supplied into the drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 mm) was used, whereas the drawing rate, 10 the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.196 N (20 gf), and 1400'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace 15 (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1450'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part 20 attaininga temperature of 1450 to 1700'C in the drawnoptical fiber is assumed to have been cooled at an average cooling rate of about 250 'C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect 25 to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.165 dB/km. The Rayleigh 60 FPOO-0085-00 scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0. 825 dB pm 4 /km. As in the foregoing, Examples 15 to 17 exhibited a 5 Rayleigh scattering coefficient of 0.815 to 0.825 dBIm 4 /km and a transmission loss of 0.163 to 0.165 dB/km with respect to light having a wavelength of 1.55 pm, thus being able to lower the Rayleigh scattering coefficient and reduce the transmission loss as compared with Example 9 yielding a 10 Rayleigh scattering coefficient of 0.825 dBpam 4 /km and a transmission loss of 0.165 dB/km with respect to light having a wavelength of 1. 55 pm. Example 17, in which the core portion of the optical fiber preform to be drawn is caused to contain Cl such that the relative refractive index difference with 15 respecttopure silicaglassbecomes 0.001, yields theresults identical to those of Example 9, so that the decrease in Rayleigh scattering intensity caused by the drop in fictive temperature due to Cl contained therein and the increase in Rayleigh scattering intensity caused by Cl itself cancel 20 out each other, whereby transmission loss will increase if Cl is contained more. Next, experimentswere carriedoutwithahigherdrawing tension while the configuration of cladding portion of optical fiber preform was changed. Examples 18 and 19 are 25 examples according to the apparatus and method for making an optical fiber in accordance with the above-mentioned 61 FP00-0085-00 second embodiment, whereas Comparative Examples 8 to 9 are comparative examples carried out for comparison with the examples according to the apparatus and method for making an optical fiber in accordance with the above-mentioned 5 second embodiment. As shown in Figs. 7A and 7B, the optical fiber preform 402 used in the following experiments is constituted by a core portion 412 made of pure silica glass (refractive index n1), a first cladding portion 422 made of fluorine-doped glass (refractive index n2), and a second 10 cladding portion 432 made of pure silica glass (refractive index n1) . The first cladding portion 422 has a region extending from the outer periphery of core portion 412 to a radius a, thus yielding an outside diameter 2a. The second cladding region 432 has a region extending from the outer 15 periphery of first cladding portion 422 to a radius d (outer periphery), thus yielding an outside diameter 2d. Employed in the experiments is the optical fiber 402 in which 2d = 35 mm. As the second cladding portion 432, highly pure silica glass may be used in place of pure silica glass. 20 Example 18 The optical fiber preform to be drawn had a core portion made of pure silica glass, and a cladding portion in which the part extending from the center of optical fiber preform up to a = 12.25 mm (a/d = 0.7) was made of fluorine-doped 25 glass whereas the part extending by a = 12.25 mm (a/d = 0.7) or greater was made of pure silica glass. Supplied into the 62 FPOO-0085-00 drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 mm) was used for drawing an optical fiber. The drawing rate, the drawing 5 tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were setto l00m/min, 0.490N (50gf), and1450'C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) 10 was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting fromtheheating furnacewas 1450'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature 15 of 1450 to 1700'C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250 'C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect 20 to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.164 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.825 dByPm 4 /km. 25 Example 19 The optical fiber preform to be drawn had a core portion 63 FP00-0085-00 made of pure silica glass, and a cladding portion in which the part extending from the center of optical fiber preform up to a = 15.75 mm (a/d = 0.9) was made of fluorine-doped glass whereas the part extending by a = 15.75 mm (a/d = 0.9) 5 or greater was made of pure silica glass. Supplied into the drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 mm) was used for drawing an optical fiber. The drawing rate, the drawing 10 tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) weresettol00m/min, 0.490N (50gf), and145 0 *C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) 15 was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1450 'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature 20 of 1450 to 1700"C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250' C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect 25 to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.166 dB/km. The Rayleigh 64 FP00-0085-00 scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.835 dBim 4 /km. Comparative Example 8 5 The optical fiber preform to be drawn had a core portion made of pure silica glass, and a cladding portion in which the part extending from the center of optical fiber preform up to a = 16.625 mm (a/d = 0.95) was made of fluorine-doped glass whereas the part extending by a = 16.625 mm (a/d = 10 0.95) or greater was made of pure silica glass. Supplied into the drawing furnace (muffle tube) was N 2 gas. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 mm) was used for drawing an optical fiber. The drawing rate, the 15 drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.490 N (50 gf), and 1450*C, respectively. In this case, the temperature of the optical fiber immediately before entering the heating furnace 20 (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1450'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part 25 attaininga temperature of 1450 to 1700'C in the drawn optical fiber is assumed to have been cooled at an average cooling 65 FP00-0085-00 rate of about 250*C/s in a segment of 2 m which is the total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pim) of the drawn optical 5 fiber was measured and found to be 0.176 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.89 dBpm 4 /km. Comparative Example 9 10 The optical fiberpreformtobe drawnhadacoreportion made of pure silica glass, and a cladding portion in which the whole area (a = d) was made of fluorine-doped glass. A heating furnace having a muffle tube with a total length (L2) of 2 m (and an inner peripheral diameter of about 30 15 mm) was used for drawing an optical fiber. Supplied into the drawing furnace (muffle tube) was N 2 gas. The drawing rate, the drawing tension, and the temperature of heating furnace (surface temperature at the inner peripheral face of muffle tube) were set to 100 m/min, 0.490 N (50 gf), and 20 1450*C, respectively. In this case, the temperature of the optical fiber immediately be fore entering the heating furnace (entering temperature) was 1700'C in terms of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating 25 furnace was 14500C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part 66 FPOO-0085-00 attaining a temperature of 1450 to 1700 "C in the drawn optical fiber is assumed to have been cooled at an average cooling rate of about 250'C/s in a segment of 2 m which is the total length of heating furnace. 5 The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.185 dB/km. The Rayleigh scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.94 10 dBPm 4 /km. As in the foregoing, Examples 18 and 19 exhibited a Rayleigh scattering coefficient of 0.825 to 0.835 dByn 4 m/km and a transmission loss of 0.164 to 0.166 dB/km with respect to light having a wavelength of 1.55 pm as with Examples 15 9 to 12, thus being able to lower the Rayleigh scattering coefficient and reduce the transmission loss as compared with Comparative Examples 8 and 9 yielding a Rayleigh scattering coefficient of 0.89 to 0.94 dBym 4 /km and a transmission loss of 0.176 to 0.185 dB/km with respect to 20 light having a wavelength of 1.55 pm. Next, an experiment was carried out while changing the optical fiber preform. Example 20 is an example according to the apparatus and method for making an optical fiber in accordance with the above-mentioned first and second 25 embodiments. The optical fiber preform employed in the following experiment has a core portion made of Ge-doped 67 FP00-0085-00 silica glass, and a cladding portion made of silica glass. Example 20 Using a heating furnace having a muffle tube (with an inner peripheral diameter of about 30mm) in which the distance 5 (Ll) from the lower end of drawing furnace to the upper end of muffle tube of heating furnace was 0.4 m and the total length (L2) of muffle tube of heating furnace was 2.0 m, an optical fiber was drawn. The gap between the drawing furnace.and the heating furnace was set to 0.05 mm. The 10 optical fiber preform to be drawn had a core portion made of Ge-doped silica glass, anda cladding region made of silica glass. The relative refractive index difference An between the core portion and claddingportion was 0.36%. The drawing rate, the drawing tension, and the temperature of heating 15 furnace (surface temperature at the inner peripheral face of muffle tube) were set to 8 m/s (480 m/min), 0.785 N (80 gf), and1400'C, respectively. Inthis case, the temperature of the optical fiber immediately before entering the heating furnace (entering temperature) was about 1500'C in terms 20 of the surface temperature of optical fiber, whereas the temperature of the optical fiber immediately after exiting from the heating furnace was 1500'C in terms of the surface temperature of optical fiber. Hence, in the heating furnace, the part attaining a temperature of 1500 to 1600'C in the 25 drawn optical fiber is assumed to have been cooled at an annealing rate of 500 to 700'C/s in a segment of 2.0 m which 68 FP00-0085-00 is the total length of heating furnace. The transmission loss (transmission loss with respect to light having a wavelength of 1.55 pm) of the drawn optical fiber was measured and found to be 0.182 dB/km. The Rayleigh 5 scattering coefficient determined from the measured data of wavelength characteristic of transmission loss was 0.92 dBym 4 /km, thus being able to fully reduce the transmission loss as a Ge-doped single-mode optical fiber. The output diameter of drawn optical fiber was 125±0.1 pam. 10 Thus, as can be seen from the results of experiments mentioned above, the cooling rate is set to 1000*C/s or less in the segment where the optical fiber 3 attains a temperature difference of at least 50 0 C in the part of optical fiber 3 yielding a temperature of 1300 to 1700*C after being drawn 15 upon heating in the drawing furnace before being coated with the UV resin 52 in the apparatus and method of making an optical fiber in accordance with the second embodiment, so that the optical fiber 3 lowers its fictive temperature, and the disorder in atomic arrangement is reduced, whereby 20 it is possible to make an optical fiber having lowered its transmission loss by reducing the Rayleigh scattering intensity within a very short period from the drawing upon heating to the coating with the UV resin 52. Also, since the Rayleigh scattering intensity is reduced by controlling 25 the cooling rate of optical fiber 3 after drawing before being coated with the UV resin 52, the heat treatment for 69 FP00-0085-00 reheating such as that in the above-mentioned prior art is unnecessary, whereby the apparatus is quite easily applicable to the mass production of the coated optical fiber 4 whose surface is coated with the UV resin 52 cured thereon. 5 When an optical fiber preform having a core portion yielding a relative refractive index difference of 0.001 or less with respect to pure silica glass in a state containing a dopant is employed as an optical fiber preform 2, the optical fiber 3 further lowers its fictive temperature, whereby the 10 Rayleigh scattering intensity can further be reduced. When an optical fiber preform whose core portion is caused to contain hydroxyl group such that the transmission loss caused by hydroxyl group absorption at a wavelength of 1.38 pm becomes 0.02 to 0.5 dB/km, the optical fiber 3 15 further lowers its fictive temperature, whereby the Rayleigh scattering intensity can further be reduced. When hydroxyl group is contained such that the transmission loss caused byhydroxyl group absorption at awavelengthof 1.38pimbecomes at least 0.02 dB/km, the optical fiber 3 further lowers its 20 fictive temperature, whereby the Rayleigh scattering intensity can further be reduced. If hydroxyl group is contained such that the transmission loss caused by hydroxyl group absorption at a wavelength of 1.38 pm is greater than 0.5dB/km, losswill increase due to the absorptionbyhydroxyl 25 group, thus canceling out the effect of lowering Rayleigh scattering intensity causedby the addition of hydroxylgroup, 70 FPOO-0085-00 whereby transmission loss will increase. When an optical fiber preform whose core portion is caused to contain Cl so as to yield a relative refractive index difference of 0.0001 to 0.001 with respect to pure 5 silica glass is employed as the optical fiber preform 2, the optical fiber 3 further lowers its fictive temperature, whereby the Rayleigh scattering intensity can further be reduced. When Cl is contained such that the relative refractive index difference with respect to pure silicaglass 10 is at least 0.0001, the optical fiber 3 further lowers its fictive temperature, whereby the Rayleigh scattering intensity can further be reduced. If Cl is contained such that the relative refractive index di f ference becomes greater than 0.001, the Rayleigh scattering intensity is enhanced 15 by Cl itself, so that the effect of lowering Rayleigh scattering intensity caused by the addition of Cl is canceled out, whereby transmission loss will increase. Using the optical fiber preform 402 having the first cladding portion 422 whose part extending from the outer 20 periphery of core portion 412 to apositionwhere the distance from the center of optical fiber preform 402 is within the range from 0.7 to 0.9 in terms of the ratio (a/d) with respect to the radius of optical fiber preform 402 is made of fluorine-doped glass, and the second cladding portion 432 25 whose part extending to the outer periphery from the position where the distance from the center of optical fiber preform 71 FP00-0085-00 402 is within the range from 0.7 to 0.9 in terms of the ratio (a/d) with respect to the radius of optical fiber preform 402 is made of pure silica glass, the Rayleigh scattering intensity is restrained from increasing even when a high 5 tension acts on the optical fiber 3, whereby the increase in loss can be suppressed. In order to prevent characteristics of the optical fiber 3 from being affected, the part where the distance from the center of optical fiber preform 402 is 0.7 or greater in terms of the ratio (a/d) 10 with respect to the radius of optical fiber 402, as the part unrelated to transmission of light, is made of pure silica glass. If the part extending to the outer periphery from the position where the distance from the center is greater than 0.9 in terms of the ratio (a/d) with respect to the 15 radius of optical fiber preform 402 is made of pure silica glass, the Rayleigh scattering intensity will change when the tension is enhanced, whereby transmission loss will increase. When the drawing rate is high, since anHe gas atmosphere 20 is attained within the muffle tube 13 of drawing furnace 11, while air-cooling is carried out between the drawing furnace 11 and the heating furnace 21, so that the part of optical fiber 3 attaining a temperature higher than 17000C before entering the heating furnace 21 is cooled at a cooling 25 rate of 4000CC/s or higher, the height of equipment required for cooling the optical fiber 3 can be lowered. At a 72 FPOO-0085-00 temperature higher than 1700*C, the structural relaxation of atoms proceeds in a very short period of time, so that the equilibrium state at each temperature can be maintained even when cooled at a cooling rate of 4000'C/s or higher, 5 whereby no influence is exerted upon the Rayleigh scattering intensity. Industrial Applicability The apparatus and method for making an optical fiber in accordance with the present invention can be utilized 10 in a drawing apparatus for drawing an optical fiber from an optical fiber preform, and the like. 73

Claims

1. An apparatus for making an optical fiber for drawing an optical fiber preform upon heating and coating thus drawn optical fiber with a resin; said apparatus 5 comprising: a heating furnace, disposed between a drawing furnace for drawing said optical fiber preform upon heating and a resin coating section for coating said drawn optical fiber with said resin, for heating said drawn optical fiber such 10 that said optical fiber attains a temperature within the range from 1200 to 1700*C; said heating furnace having a muffle tube through which said drawn optical fiber passes; said muffle tube being disposed at a position 15 satisfying: L1s0.2xV where L1 is the distance (m) from the lower end of a heater in the drawing furnace to the upper end of said muffle tube; 20 and V is the drawing rate (m/s).

2. An apparatus for making an optical fiber according to claim 1, wherein said heating furnace heats said drawn optical fiber at a temperature within the range 25 from 1200 to 1600 0 C.

3. An apparatus for making an optical fiber 74 FP00-0085-00 according to claim 1, wherein said muffle tube is disposed at a position where said drawn optical fiber attains a temperature within the range from 1400 to 18000C when entering said muffle tube. 5

4. An apparatus for making an optical fiber according to claim 1, wherein said muffle tube is formed so as to satisfy: L22V/8 where 10 L2 is the total length (m) of said muffle tube; and V is the drawing rate (m/s).

5. An apparatus for making an optical fiber according to claim 1, wherein saidheating furnace is provided with such a temperature gradient that the drawing furnace 15 side and the resin coating section side attain higher and lower temperatures, respectively.

6. A method of making an optical fiber comprising the steps of drawing an optical fiber preform upon heating and coating thus drawn optical fiber with a resin; 20 wherein a segment of said optical fiber yielding a temperature difference of at least 500C in a part where said optical fiber before being coated with said resin attains a temperature of 1300 to 17000C is cooled at a cooling rate of 1000*C/s or lower. 25

7. A method of making an optical fiber according to claim 6, wherein an optical fiber preform having a core 75 FPOO-0085-00 portion exhibiting a relative refractive index difference of 0.001 or less with respect to pure silica glass in a state containing a dopant is employed as said optical fiber preform; and 5 wherein said optical fiber preform is drawn upon heating.

8. A method of making an optical fiber according toclaim7, wherein anoptical fiberpreformwhosecoreportion is caused to contain hydroxyl group so as to yield a 10 transmission loss of 0.02 to 0.5 dB/km due to hydroxyl group absorption at a wavelength of 1.38 pm is employed as said optical fiber preform; and wherein said optical fiber preform is drawn upon heating. 15

9. A method of making an optical fiber according toclaim7, wherein anoptical fiberpreformwhosecoreportion is caused to contain Cl so as to yield a relative refractive index difference of 0.0001 to 0.001 with respect to pure silica glass is employed as said optical fiber preform; and 20 wherein said optical fiber preform is drawn upon heating.

10. A method of making an optical fiber according to claim 7, wherein an optical fiber preform having a cladding portion whose part extending to the outermost periphery from 25 a position where the distance from the center of said optical fiber preform is within the range from 0.7 to 0.9 in terms 76 FPOO-0085-00 of the ratio with respect to the radius of said optical fiber preform is made of highly pure silica glass is employed as said optical fiber preform; and wherein said optical fiber preform is drawn upon 5 heating..

11. A method of making an optical fiber according to claim 7, wherein a part of said optical fiber having a temperature higher than 1700*C before being coated with said resin is cooled at a cooling rate of 4000*C/s or higher. 10

12. An apparatus for making an optical fiber for drawing an optical fiber preform upon heating and coating thus drawn optical fiber with a resin; said apparatus comprising: a heating furnace, disposed between a drawing furnace 15 for drawing said optical fiber preform upon heating and a resin coating section for coating said drawn optical fiber with said resin, for heating said drawn optical fiber such that a segment of said optical fiber yielding a temperature difference of at least 50*C in a part of said optical fiber 20 attaining a temperature of 1300 to 1700*C is cooled at a cooling rate of 1000 0 C/s or less.

13. An apparatus for making an optical fiber according to claim 12, further comprising atmosphere gas supplying means for supplying, as an atmosphere gas of said 25 optical fiber within said heating furnace, an atmosphere gas having a thermal conductivity on a par with or lower 77 FPOO-0085-00 than that of an atmosphere gas in said drawing furnace.

14. An apparatus for making an optical fiber according to claim 12, further comprising: outside diameter measuring means for measuring the 5 outside diameter of said optical fiber having exited from said heating furnace; and control means for controlling the drawing rate of said optical fiber according to a result of measurement by said outside diameter measuring means such that the outside 10 diameter of said optical fiber attains a predetermined value. 78